Cochlear implants and other implantable medical devices are commonly powered from a battery included in an external device (e.g., a speech processor apparatus) through a radio frequency (“RF”) inductive link. To minimize the size of the external device and to maximize battery life, an RF power transmitter included in the external device needs to be physically small, electrically efficient, and adjustable in small increments in order to optimize the power delivery for the stimulation requirements at any given moment. In particular, cochlear implants require frequent and fine resolution power adjustments to maximize power efficiency within the users' dynamically changing audio environments.
In some conventional cochlear implant systems, the RF power transmitter consists of two parts. A class-D power amplifier is implemented in a CMOS integrated circuit, and this provides small size and good efficiency. However, in order to provide fine resolution power adjustments (e.g., 256 steps), an adjustable inductive switching regulator is used, which requires a separate integrated circuit and a large off-chip inductor. This has a negative impact on the overall size of the RF power transmitter, and hence, the external device.